1. Field of the Invention
The present invention pertains to measuring an electric potential signal generated by an object and, more particularly, to reducing the effect of artifact signals or signals caused by a change in static potential on measurement of an electric potential signal of interest.
2. Discussion of the Prior Art
Capacitive electrodes that sense the electrical potential produced in the space surrounding a voltage source enable many important new measurement modalities. For example, a particularly important new capability is the measurement of bioelectric signals, such as those measured during an electrocardiogram (ECG) or an electroencephalogram (EEG), without touching a subject's skin. In such cases, signals of interest, such as those produced by a heart or brain, are measured. An arrangement for making such measurements is set forth in U.S. Pat. No. 6,686,800 which is hereby incorporated by reference. While the system set forth in the '800 patent is effective, almost all capacitive measurements that occur in real world environments are degraded by artifact signals and noise caused by the presence of static electric fields.
Static electric fields are everywhere. An important limiting factor in capacitive measurements is the behavior of static electric potential in the region where sensors must be located to measure a signal of interest. If ambient static potentials where truly static and never changed, they could not be measured by sensors using a capacitive electrode arrangement. However, the static potential on a measured object changes for various reasons. For example, the static potential level may rise dramatically due to triboelectric charging and reach levels on the order of 10,000V. Further, the static potential can vary significantly due to discharging and the influence of other conducting bodies. In addition, motion of a capacitive sensor relative to the static electric field produces a change in the potential that is sensed. Such changes in the sensed potential can saturate the capacitive electrode causing large changes in the output signal produced by the sensor. Even at low levels, changes in the static electric field will appear as measured unwanted artifact signals. Such artifact signals can often be very similar in frequency to the signal of interest and thus are typically very difficult to filter.
In the case of a sensed bioelectric signal, certain artifact signals, such as common mode signals, can be particularly troublesome. Often, common mode signals are of low frequency and can be read as a heartbeat. Such a state of affairs can lead to the measuring equipment reading a normal heartbeat when, in fact, the heartbeat is not normal. The opposite is also true and can lead to measuring equipment reading an irregular heartbeat or fibrillation when the heartbeat is actually normal. One solution to this problem is suggest by Leyde et al. in U.S. Pat. No. 5,650,750 which is hereby incorporated by reference. Leyde et al. simply propose a way to measure faults. To this end, in the Leyde et al. arrangement, common mode voltage signal is measured and identified. However, no simple way of eliminating such signals is proposed.
In the case where an electric potential is measured through a more common resistive type coupling in which a low-impedance electrical contact is made with the object of interest, the static potential on the object is relatively easy to control. Typically it is controlled with a simple additional electrical connection to the object. For example, an electrocardiogram (ECG) taken in a hospital involves electrodes that attach to a patient's skin via a gel or adhesive that make electrically conducting connections with the patient's body. With such an arrangement, it is relatively simple to discharge static electricity by using an additional resistive contact ground strap, or even to use the electrodes themselves. However, the main idea behind using capacitive electrodes is to measure signals in a noninvasive manner. Since, by its very nature, a non-invasive capacitive measurement system cannot make a resistive coupled electric contact with an object, in such a system, a static potential cannot be reduced in this manner.
One proposed method to facilitate capacitive measurement is to allow an entire measurement system to float at the potential of the object. Such an approach is designed to minimize the absolute potential presented to the sensor relative to the remainder of the sensing system. However, this approach is limited by how similar the potential of the sensing system is to the static potential of the object. For a measuring system that is only connected to an object of interest by a capacitive coupling, the measuring system's potential is always different from that of the object, thereby producing a potential difference across the sensing region that could still be rather large compared to the signal of interest.
Yet another proposed solution to reducing signal artifacts when using capacitive measuring to sense a voltage signal of interest is to use two or more capacitive electrodes and subtract their outputs. Taking the difference of the two sensor measurements is standard practice in many applications. For example, the lead configurations used to diagnose heart ailments in an ECG have such a configuration. So long as the static potential does not exceed the measurement range of the sensors, the effect of such static potential can be removed for a large extent by taking the difference between two or more sensor inputs. Such an approach works relatively well for conventional skin contact resistive electrodes but does not work so well for capacitive electrodes, particularly ones that are not firmly attached to the body.
A capacitive-based system requires measuring a relatively small signal of interest, such as a heartbeat, against the background of much larger signals. Essentially, a small signal would have to be determined by subtracting the measured value of two or more relatively large static potential signals. This type of measurement would be limited by the unknown variations in the coupling of each sensor to the static potential, the calibration precision of the sensor components, and the dynamic range of the differencing system. The calibration precision of the sensor components and the dynamic range of the differencing system can be improved through increased complexity of the measuring system and increased cost associated with improving the quality of the measuring system's parts. However, the unknown variations of the coupling of each sensor is a fundamental problem for capacitors in a static electric field. Of course, the variations in coupling to the static electric potential can be minimized by positioning the sensors firmly against the measured object. However, such an approach is not compatible with non-invasive measuring. For example, such a technique would not work with sensors located in clothing.
There is inherently a lack of precision and repeatability in the coupling efficiency of a capacitive sensor to the static potential due to changes in the capacitance electrode that couples to the potential. If the fraction of the static signal coupled to two different types of sensors is different, then subtracting their outputs will not cancel the static signal even if the sensors are identical. The variation in coupling efficiency can be understood by the following example. Say a signal presented at the input of a first stage amplifier of a capacitor sensor is determined by a potential divider network comprised of an electrode capacitance and an input capacitance of an amplifier. On a human body, variations in the electrode capacitance can occur due to variations in the thickness of the outer skin layer, variations in the thickness of the clothing between the electrode and the subject, and a relative motion between the electrode and the subject.
Based on the above, there exists a need in the art for a system that is able to reduce the effect of a background static electric field when making high-sensitivity capacitive measurements of the electric potential in the vicinity of an object of interest, while not interfering with the overall goals of a non-invasive measuring system. Another aspect of the invention is to make the signal measured by a capacitive electrode less susceptible to the differences in electrode capacitance.